ASHRAE Journal - April 2010 - 32

Standard 62.1 includes default values for occupant density. A note included under Table 6-1 clarifies that design engineers are not required to use this default, but may choose to if the actual occupant density is not known. Using an overly conservative estimate for zone population can result in significant overventilation and excessive energy use. It also impacts the calculation of the space latent cooling load, and the resulting dehumidification capacity of the dedicated OA unit. The recommendation of this author is to avoid applying safety factors to zone population (Pz), instead using the best estimate for expected occupancy during typical usage. Then, select the dedicated OA unit with “reserve” airflow capacity, as will be explained later in this article.
Safety Factor for Future Expansion or Change of Use

Cool + Reheat
OA C R C H CA

Active, Heat-Regenerated Desiccant
H OA Desiccant
Wheel Active

C C CA

Cool + Reheat With Exhaust-Air Energy Recovery
OA EA EA
Total Energy Wheel C

Series Type III Desiccant Wheel With Exhaust-Air Energy Recovery
Series Desiccant Wheel Total Energy Wheel

R C H CA C C

OA EA

Figure 2: Example dedicated outdoor air equipment types.

The other common use of a safety factor when e determining the design airflow of the dedicated OA ur at er p unit occurs when accounting for future expansion m Te lb qL or a change in use of the facility. If the facility is -Bu t We expanded in the future, and the dedicated OA unit will be expected to serve the expansion, it would be prudent to select a unit with some amount of reserve (or extra) airflow capacity. Or, if the use of the facility is likely to change in the future (from an office space to a group of meeting rooms or a Dry-Bulb Temperature °F retail area, for example) the ventilation requirements for the zones served by the dedicated OA Figure 3: Dehumidification provided by a chilled water or direct-expansion dediunit may change. Again, it would be prudent to cated outdoor air unit. select a unit with reserve airflow capacity. The recommendation of this author is to calculate the design Since the previous section discussed the influence of safety airflow as accurately as possible, without using safety factors. factors on the design airflow (Vot), this section will focus on Then select a dedicated outdoor air unit that has reserve airflow the remaining two variables in Equation 3: the humidity ratio capacity, rather than a unit operating near its maximum allowable of the entering outdoor air (Woa) and the humidity ratio of the airflow. The impact of this will be discussed later in this article. leaving conditioned air (Wca).
Determining Design Dehumidification Capacity

The required dehumidification capacity of a dedicated OA unit (Figure 3, Equation 3) is dictated by the design airflow (Vot), humidity ratio of the entering outdoor air (Woa), and humidity ratio of the conditioned air leaving the unit (Wca): qL = 4.5 × Vot × (Woa – Wca) / 7000 gr/lb (3)

where qL = required dehumidification capacity, lb/h Vot = design outdoor airflow, cfm Woa= humidity ratio of the entering outdoor air, grains/ lb Wca= humidity ratio of the leaving conditioned air, grains/lb
32 ASHRAE Journal

ASHRAE Handbook—Fundamentals7 is a popular source for climatic data that represents the outdoor design conditions for various locations. Historically, design engineers have used the design dry-bulb temperature and mean coincident wet-bulb temperature when calculating the required capacity of cooling systems. However, the highest outdoor humidity ratio does not occur at the same time as the highest outdoor dry-bulb temperature. Since 1997, the ASHRAE Handbook has included the design dew-point temperature and mean coincident drybulb temperature for use when calculating the required capacity of dehumidification systems. Table 1 lists the 0.4% outdoor design conditions for Jacksonville, Fla. 7 Notice that the outdoor humidity ratio is 32% higher at the design
ashrae.org April 2010

Safety Factor for Woa (Humidity Ratio of the Entering Outdoor Air)

Humidity Ratio, grains/lb of Dry Air

°F

ASHRAE Journal - April 2010

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